SE510740C2 - Oxygen delignification control - Google Patents

Abstract

Methods of controlling oxygen delignification processes are disclosed including providing chemicals for a first oxygen delignification step, conducting the first oxygen delignification step with the chemicals, conducting a second oxygen delignification step at a predetermined temperature to produce a delignified pulp, measuring the final kappa number of the delignified pulp subsequent to the second oxygen delignification step, adjusting the chemicals provided for the first oxygen delignification step based upon the difference between the actual kappa number reduction and the desired kappa number reduction for the pulp, and adjusting the predetermined temperature based on the chemicals provided to the first oxygen delignification step in order to provide the delignified pulp at a final pH of from about 10.5 to about 11.5.

Description

510 740 2 the first delignification step is performed. From there, the pulp is optionally conveyed by means of a second pump 4 via a second mixer 5 for mixing steam and possibly additional oxygen and alkali to a second reactor 6 for the second delignification step. After the second reactor 6, the pulp is transferred to a blowing tank 7 and on to subsequent treatment steps.

The process thus means that the delignification is carried out in two successive steps. In the first mixer 2, both a high alkali additive and a high oxygen additive are made.

This means an investment of 10-50 kg of alkali (NaOH) per tonne of pulp, preferably 10-35 kg / tonne. Optionally, this required alkali batch can be obtained in part by transfer from the brown pulp wash. The investment in mixer 2 can then be reduced to a corresponding degree. The oxygen charge should be 10-50 kg / ton pulp, preferably 10-30 kg / ton.

The temperature of the pulp at the feed into the reactor 3 should be below 90 ° C, preferably 75-90 ° C. The residence time in the reactor 3 should be relatively short, 5-30 minutes, preferably 15-25 minutes.

In the first reactor 3, the pressure should be 4-15 bar.

Together with the high alkalinity of the pulp and the high oxygen concentration, the high pressure results in a high delignification rate. At the same time, the rate of cellulose degradation is kept at a low level due to the relatively low temperature and short residence time.

After the first delignification step in the first reactor 3, the pulp is transferred to the second delignification step in the second reactor 6. The temperature in the second reactor 6 should be higher than in the first reactor 3. However, the temperature difference should be less than 20 ° C, preferably 10-15 ° C. To achieve the required temperature increase, steam is supplied to the second mixer 5. The pressure in the second reactor 6 should be 2-5 bar and lower than in the first reactor 3. The residence time should be relatively long, 45-180 minutes, preferably 60-120 min.

The second delignification step is mainly a long extraction step where, in relation to the first step, the elevated temperature and the extended residence time give extended delignification. At temperatures above 90 ° C, good extraction / leaching rate is thus obtained. The main part of the chemical investment is made in the first step. Preferably very little or no additional alkali or oxygen charge should be made in the second stage, not even to compensate for the consumption in the first stage.

The alkalinity of the pulp can then be kept relatively low in the second stage. This mainly avoids cellulose degradation despite high temperature and long residence time.

The charge of alkali or oxygen can be up to 5 kg / tonne of pulp.

The control of the oxygen delignification according to the invention is based on forward control, ie it involves the least possible feedback. The control is performed as follows.

The kappa number of the incoming pulp is measured and compared with a desired value (setpoint) for the kappa number of the pulp after the oxygen delignification. The so-called reduction of the kappa number is used to control the chemical investment (oxygen / alkali) to the first stage. Larger kappa number reduction gives a higher investment calculated per reduced kappa number unit. Furthermore, the level of the incoming mass kappa number is also used to control the chemical investment so that higher incoming kappa numbers result in a lower investment calculated per reduced kappa number unit.

The temperature level in the second stage is controlled partly in the usual way by the production level, so-called coat factor control, and partly by the chemical investment to the first stage so that a higher chemical investment leads to a higher temperature in the second stage. The temperature is controlled so that a final pH of 10.5-11.5 is obtained.

On the other hand, there is normally no compensation of the temperature in the first step towards the chemical investment and the production level. The mass entering the first stage is not heated. However, it may need to be cooled slightly to obtain a low temperature suitable for the process.

Higher degrees of delignification, ie greater kappa number reduction, means a lower ratio between the oxygen and alkali investment.

As a result, duct formation due to large amounts of gas can be avoided, among other things. The limit can be at an oxygen investment of 25-30 kg oxygen per tonne of pulp.

The control of oxygen delignification according to the invention means that a high degree of delignification with good selectivity can be achieved. A low and even kappa number as well as an even pH value can be obtained after the second delignification step.

Claims (2)

° 51o 740 Patent claim 1. A method for controlling oxygen delignification of pulp, i.e. lowering of the pulp kappa number from an incoming kappa number, wherein the delignification is carried out in two steps with the addition of the main part of the chemicals required for the delignification to the first stage, characterized in that kappa number during the delignification is used to control the chemical investment to the first stage, where a larger kappa reduction requires a higher chemical investment calculated per reduced kappa unit, and that the chemical investment is in turn used to control the temperature in the second stage so that a final pH of 10.5 11.5 is obtained. 2. A method according to any one of claims 1, characterized in that the entire chemical investment required for the delignification is made in the first step.